Colour rendition

The quality of how colors are reproduced is called color rendering. Line spectra have very good color rendering. Line spectra allow only a single color to be perceived well. Multi-line spectra render several colors of the respective spectrum well, but color rendering is weaker in the intermediate ranges. Blue and green colors appear comparatively gray and dull in warm white incandescent light despite excellent color rendering. However, these color tones appear clear and luminous in the light of daylight white fluorescent lamps – despite poorer color rendering. With the rendering of yellow and red color tones, this phenomenon of weakening or intensifying of the color effect is reversed.

Because the eye is able to adapt to light of the most different color temperatures, the color rendition must be determined dependent on the color temperature. Tungsten halogen lamps feature very good color rendition. The rendition quality of fluorescent lamps and metal halide lamps ranges from good to average. The degree of color distortion against a reference light source is indicated using the color rendition index Ra or the color rendition grading system. The color rendition index is only used for white colors of light.

Color rendering describes the change in the color impression under a given illumination compared to a reference light source. The general color rendering index CRI is usually used for evaluation purposes – the maximum value of 100 means no change. A comparable light source with a continuous spectrum is used as a reference light source, whether this is a thermal radiator of comparable color temperature or daylight.

To enable the color rendition of a light source to be determined, the chromatic effects of a scale of eight body colors viewed under the type of illumination being scrutinized and also under the reference illumination are calculated and related to each other. The resulting quality of color rendition is expressed in color rendition indices; these can relate both to the general color rendition (Ra) as an average value or to the rendition of individual colors. The maximum index of 100 signifies ideal color rendition as experienced with incandescent lamp light or daylight. Lower values refer to a correspondingly worse color rendition. Linear spectra of light lead to good color rendition. Linear spectra in general lead to a worse rendition. Multiline spectra are composed of several different linear spectra and improve the color rendition.